implemented keepOpen and item refresh after click

Implemented two features:

1. If a menu item includes the `keepOpen` option which evaluates to `true`, the menu is not closed after an item is clicked.

2. If a menu is kept open, the clicked item is refreshed by reapplying the current values of `iconImage` and `iconCssClass`. This way we can dynamically change the appearance, e.g. to mark that some items were filtered out.